As a structure for mutually connecting wiring patterns which are located in desired layers of a board having two or more wiring layers, there is a structure called a through hole or a via (or via hole). Particularly, a structure to connect inner layers of a multilayer board using a hole is called a via.
FIG. 1 is a diagram schematically illustrating a multilayer board including a via. FIG. 1 shows a four-layer structure including layer 1 which is ground layer 11, layer 2 which is signal line 12, layer 3 which is signal line 13, and layer 4 which is ground layer 14. Via 15 connects the signal lines of layer 2 and layer 3. Meanwhile, signal line 12 is connected to, for example, a high-frequency amplifier, and signal line 13 is connected to, for example, an antenna.
Generally, a via is formed of a complicated shape, and impedance mismatching occurs in a connecting part between a signal line having a constant line width and a via. For this reason, it is known that signal transmission characteristics, particularly, at the time of high-frequency driving deteriorate. That is, the reflection and attenuation of a signal occurs in the connecting part between the signal line and the via.
In this respect, PTL 1 proposes a method for improving the electrical characteristics of the via structure. For example, PTL 1 proposes a method for performing pseudo-coaxialization on vias provided in the vertical direction of an insulating board, using a plurality of ground layers lined up on the insulating board located in the vicinity of the vias. In addition, PTL 1 proposes a multilayer board including stacked layers and having a structure in which coaxial via holes spaced apart at regular intervals are formed in the vicinity of the vias by laser processing, and the coaxial via holes are connected to a power supply and ground layer.
Further, as shown in FIG. 2, PTL 1 proposes a multilayer wiring board in which insulating layers each formed using a resin insulating film and wiring layers each formed using a conductor film are alternately stacked one on top of another. This multilayer wiring board includes via holes which are connected to signal transmission wiring and via holes which are independently formed in the vicinity of the vias coaxially on concentric circles separated at a regular interval by the insulating layer without being connected to a signal wiring layer, a power supply and a ground layer, and are buried in the insulating resin layer.